As ND filters (neutral density filters), a reflection type ND filter which reflects incident light to attenuate it and an absorption type ND filter which absorbs incident light to attenuate it are known in the art. Then, where an ND filter is set in a lens optical system in which the reflected light comes into question, the absorption type ND filter is commonly used. This absorption type ND filter includes a type in which the substrate itself is incorporated with an absorptive substance (a color glass ND filter) or coated with it, and a type in which the substrate itself does not absorb light and a thin film formed on its surface does. Also, in the case of the latter, in order to prevent reflection at the thin-film surface, the thin film is constituted of a multiple layer (an absorption type multi-layer film) so that it can have the function to attenuate transmitted light and also have the effect of preventing the reflection.
Now, in an absorption type multi-layer film ND filter used in compact thin-type digital cameras, its substrate itself must be thin because the camera has a narrow set-in space. Accordingly, a resin film is used as an optimum substrate.
As the absorption type multi-layer film ND filter whose thin film is constituted of a multiple layer, Japanese Patent Application Laid-open No. H05-93811 discloses an ND filter consisting of an oxide dielectric film layer and a metal layer, where the metal layer is used as an absorption film layer.
An ND filter is also known in which, as the absorption film layer, a metal oxide film of TiOx, Ta2Ox or the like is employed at the time of film formation of which oxygen is intentionally fed in to make it have absorption due to oxygen deficiency.
Here, where the absorption film layer is made up of the metal film at the time of film formation of which oxygen is not intentionally fed in, such a metal film has a higher extinction coefficient (absorptivity coefficient) than the metal oxide film of TiOx, Ta2Ox or the like, and hence, in order to achieve the like extinction coefficient, the employment of the metal layer enables formation of the absorption film layer in a smaller layer thickness.
Then, where the absorption type multi-layer film is formed on a resin film substrate having a flexibility, it is more advantageous to employ as the absorption film layer such a metal layer that enables its layer thickness to be set smaller than that of the metal oxide film, taking account of warpage of the resin film substrate, cracking of films, time for film formation, and so forth.
However, the metal film and the metal oxide film of TiOx, Ta2Ox or the like not completely oxidized may easily proceed with oxidation to come to have a low extinction coefficient, and hence an ND filter employing the metal film or the metal oxide film not completely oxidized comes to have a higher transmittance with time, as so known in the art. This has been remarkable especially in the case of the metal film.
Thus, in the ND filter employing the metal film or the metal oxide film not completely oxidized, it has come into question that the absorption film layer proceeds with oxidation in a high-temperature and high-humidity environment to come to increase in its transmittance.
Now, the oxygen that oxidizes the metal film or the metal oxide film not completely oxidized is considered to be fed from the atmosphere or from the resin film substrate or the oxide dielectric film layer. In particular, the metal layer tends to be affected by the oxidation if it is 10 nm or less in thickness.
Accordingly, in order to prevent oxidation of the metal film or the like, a method is proposed in which heat treatment is carried out in the atmosphere or in an atmosphere of oxygen so as to oxidize the metal film or the like at its interface and in the vicinity thereof, and not to oxidize the metal film or the like up to its interior. For example, in Japanese Patent Application Laid-open No. 2003-43211, a thin-film ND filter is proposed in which a light absorbing film and a dielectric film are formed on a transparent substrate in layers, where as the light absorbing film a light absorbing film containing a metallic-material oxide TiOx is used which is formed by vacuum deposition using a metallic material Ti as a raw material, and by feeding at the time of film formation a mixed gas containing oxygen and in the state the degree of vacuum is kept constant between 1×10−3 Pa and 1×10−2 Pa. Then, the light absorbing film and the dielectric film are formed on the transparent substrate in layers, and thereafter these are heated in an atmosphere of oxygen containing 10% or more of oxygen to make any changes in optical characteristics saturated. In Japanese Patent Application Laid-open No. 2003-322709, a method is also proposed in which, in regard to a thin-film ND filter in which at least one transparent dielectric film and light absorbing film are formed on a light-transmitting substrate in layers, a lower metal nitride film layer that can not easily come to have a high transmittance as a result of oxidation is employed as the light absorbing film.
Now, the above method in which heat treatment is carried out in the atmosphere or in an atmosphere of oxygen so as to oxidize the metal film or the like at its interface and in the vicinity thereof has had a problem that the oxidation may completely proceed up to its interior especially in the case of a thin metal film of 10 nm or less in thickness to make it difficult to form an oxide film only at the interface and in the vicinity thereof. In the case when the heat treatment is carried out in the atmosphere or in an atmosphere of oxygen, it has also come about that the film comes to warp or crack because of a difference in coefficient of thermal expansion between the absorption type multi-layer film formed and the resin film substrate. Further, as the substrate resin film, a film of continuous length is usually used. Accordingly, there has been a problem that very large-scale equipment is required in order to carry out uniform heat treatment while a resin film on which the absorption type multi-layer film has been formed by means of, e.g., a sputtering roll coater is wound up around a wind-up roll.
The method employing the lower metal nitride film layer that can not easily come to have a high transmittance as a result of oxidation has resolved the disadvantage of coming to increase in transmittance, but has had a problem that the lower metal nitride film layer has a large layer thickness because of its smaller extinction coefficient than the metal film layer.
The present invention has been made taking note of such problems, and a subject of the present invention is to provide an absorption type multi-layer film ND filter having a superior environmental resistance which makes use of a resin film as a substrate, and a process for producing the same.
Accordingly, the present inventor has made extensive studies in order to resolve the above subject. As the result, he has come to discover such a means for resolution as described below.
First, when the metal film constituting the absorption film layer of the absorption type multi-layer film is oxidized in a high-temperature and high-humidity environment, oxygen is fed from the oxide dielectric film layer of the absorption type multi-layer film. Accordingly, the metal film can be kept from proceeding with oxidation if the level of oxygen contained in the oxide dielectric film layer is made lower so that the oxygen can not easily be fed. However, the oxide dielectric film layer may slightly be colored unless it stands oxidized sufficiently, and hence it can not be used in any reflection preventive films, reflecting films and so forth required to be transparent films.
However, a metal film having absorption at first is used in the absorption type multi-layer film ND filter, and hence it does not especially come into question if the oxide dielectric film layer stands slightly colored and has absorption.
It is then considered that, as long as the oxide dielectric film layer has absorption only slightly, it is unnecessary to take account of the extinction coefficient of the oxide dielectric film layer when the absorption type multi-layer film is film-designed and, if on the other hand the absorption of the oxide dielectric film layer is so large as to greatly affect the transmittance of the ND filter, the extinction coefficient of the oxide dielectric film layer may be taken into account when the absorption type multi-layer film is film-designed. The present invention has been accomplished through such technical studies.